The Czochralski method (CZ method), in which a silicon single crystal is grown and pulled from a raw material melt in a quartz crucible, has been widely implemented as a method for manufacturing a silicon single crystal for use in fabrication of a semiconductor device. In the CZ method, a seed crystal is dipped into the raw material melt (silicon melt) in the quartz crucible under an inert gas atmosphere, and the seed crystal is pulled while the quartz crucible and the seed crystal are rotated so that a silicon single crystal having a desired diameter is grown.
In recent years, grown-in defects in silicon wafers becomes an issue as higher integration of semiconductor devices and accompanying shrinking feature sizes are advanced. The grown-in defect is a factor in deteriorated characteristics of semiconductor devices and the advancement of the shrinking feature sizes of a device is heightening the effect of the defect. An octahedral void-type defect, which is an agglomeration of vacancies (See Non-Patent Document 1), and a dislocation cluster, which is formed as an agglomeration of interstitial silicon (See Non-Patent Document 2), in a silicon single crystal by the CZ method are known as such grown-in defects.
Non Patent Document 3 discloses that the amount of the grown-in defects being introduced depends on a temperature gradient at a crystal-growth interface and a growth rate of a silicon single crystal. As a method for manufacturing a low defect silicon single crystal by utilizing the dependency, for example, Patent Document 1 discloses making the growth rate of a silicon single crystal slower, and Patent document 2 discloses that a silicon single crystal is pulled at a rate less than the maximum pulling rate that is substantially proportional to a temperature gradient at a boundary region between a solid phase and a liquid phase of the silicon single crystal. Moreover, an improved CZ method focusing attention to the temperature gradient (G) and the growth rate (V) during crystal growth has been reported (Non Patent Document 4). In this method, it is necessary to control the crystal temperature gradient with high precision.
In these methods, a structure in cylindrical form or inverted cone form for shielding radiant heat (heat shielding member) is provided around the silicon single crystal being grown above a melt surface to control the crystal temperature gradient. The structure enables the crystal temperature gradient of a high temperature crystal to increase and thus brings an advantage in that a defect-free crystal can be rapidly obtained. For precise control of the crystal temperature gradient, however, it is necessary to control highly precisely such that a distance between the lower end surface of the heat shielding member located above the surface of the raw material melt and the surface of the raw material melt (hereinafter, also referred to as DPM: Distance from the purge tube to the melt surface) is brought to a predetermined distance.
However, it is difficult to control the DPM precisely so as to maintain a predetermined distance by a conventional method.
As the crystal diameter increases, the position of the melt surface widely varies in dependence on, for example, the weight (variation in thickness), deformation, and expansion in its operation of the quartz crucible. Thus, there arises a problem in that the position of the melt surface varies every crystal growth batch. Because of the problem, controlling the distance between the melt surface and the heat shielding member precisely so as to maintain a predetermined distance becomes more difficult.
For improvement of the problem, for example, Patent Document 3 proposes that a criterion reflector be provided in a CZ furnace to measure the distance between the criterion reflector and the melt surface by measuring a relative distance between a real image of the criterion reflector and a mirror image of the criterion reflector reflected on the melt surface. In Patent Document 3, the distance between the melt surface and the heat shielding member is controlled precisely on the basis of the measurement result so as to maintain a predetermined distance.
Moreover, Patent Document 4 discloses a method for stabilizing the mirror image of the criterion reflector by taking account of a curve of the raw material melt due to rotation of the crucible.
Patent Document 5 discloses a method for improving a positional detection error by applying a magnetic field to reflect an image clearly.
In these methods, the real image of the criterion reflector and the mirror image of the criterion reflector are captured with a detection means such as an optical camera, and light and darkness of the captured real image and mirror image of the criterion reflector are quantized (binarization) into two levels on the basis of a predetermined threshold (threshold for a binarization level). That is, the light and darkness are distinguished by a lighter part or a darker part than the threshold for a binarization level. The position of the edge between them is measured and the measured value is converted so that the distance from the real image or the mirror image is measured.
However, there is a problem in that precise measurement of the distance between the criterion reflector and the melt surface cannot be ensured. For example, with the passage of time in a crystal growth step, the brightness of the mirror image of the criterion reflector reflected on the melt surface varies, and the value detected by the optical camera varies before the binarization, or noise differing from that of the mirror image of the criterion reflector, such as a scattered melt attached to a structural part in the CZ furnace, is detected.
Meanwhile, when the raw material melt is contained in a quartz crucible having a diameter of 800 mm or more, for example and a silicon single crystal having a diameter of 300 mm or more is manufactured without applying the magnetic field, there is another problem in that, since the melt surface vibrates, precise detection of the position of the melt surface cannot be ensured. In this case, also, precise measurement of the relative distance between the criterion reflector and the melt surface cannot be ensured.
When the measurement result of the relative distance between the criterion reflector and melt surface is inaccurate, the distance between the melt surface and the heat shielding member cannot be controlled precisely so as to maintain a predetermined distance. As a result, a silicon single crystal with desired quality cannot be manufactured at good productivity.